German Patent Application No. 43 42 731 describes a gas sensor with a tubular finger-shapes sensor element in which one of the printed conductors running on the outside of the tubular sensor element is covered by an electrically insulating layer formed by a mixture of a crystalline, non-metallic material and a glass-forming material, a glazing, filled with the crystalline non-metallic material being formed upon heating.
Furthermore, German Patent Application No. 29 07 032 corresponding to U.S. Pat. No. 4,294,679), for example, describes a planar sensor .a element for determining the oxygen level in gases, in which a measuring cell is connected to a resistance heating element via an Al2O3 insulating layer. The ceramic heater insulation made of Al2O3 is electrically insulating and is used porously sintered to compensate for the different sinter contractions and different thermal expansion coefficients of Al2O3 and the adjacent ZrO2 solid electrolyte layer. This, however, has the disadvantage that gaseous and liquid components diffuse from the exhaust gas into the reference atmosphere through the porous insulation layer and thus affect the measuring signal. In addition, components of the exhaust.
The gas sensor according to the present invention has the advantage that the insulation layer is gas-tight and has a good electrical insulation capability, good adhesion to the solid electrolyte ceramic, and good heat conductivity. The good adhesion results, in particular, from the fact that shrinkage of the insulation layer material due to sintering is approximately equal to that of the solid electrolyte ceramic material. The compression stresses arising in the insulation layer due to the different thermal expansion coefficients of the insulation layer and the solid electrolyte foil are reduced in part by the plastic deformation due to the softening characteristics of the glass phase and uniformly distributed over the boundary surface with the solid electrolyte ceramic. Thus local stress concentrations that might cause cracks are fully avoided. The glass materials used have an initial softening temperature that is lower than the 1250xc2x0 C sintering temperature. The powder mixture used in the process for manufacturing the sensor element has proved to be particularly well-suited. The paste produced with the powder mixture is particularly well-suited for screen printing of the gas-tight insulation layers.
The particular the properties regarding gas-tightness and heat conductivity are achieved if Al2O3 with a particle size of d50 less than 0.40 xcexcm is used as the crystalline, non-metallic material. Gas-tightness of the insulation layer is further improved when a particle size distribution of d90 less than 1 xcexcm is set. With this particle size and particle size distribution, a gas tightness 2 to 4 times greater than is achievable with conventional ceramic layers can be achieved. d50 denotes the average particle size referred to the mass; d90 denotes the particle size with 90% of the mass being finer or the same. By suitable selection of particle size and particle size distribution of materials B and C in the following table, the sintering temperature can be reduced from 1600xc2x0 C. to 1250xc2x0 C. The melting point of the glass-forming material, with which a glazing filled with a crystalline, non-metallic material, for example, Al2O3, is formed, is the limit for the sintering temperature. An insulation layer that is particularly well-suited for heater insulation is achieved with a proportion of 60 wt. % of crystalline non-metallic material to 40 wt. % of glass-forming material in the raw material mixture.